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Mathias Westlund Photonics Laboratory, MC2 Chalmers University of Technology, Feb, 2005 Original version: Bamdad Bakhshi, Feb 2000.
FO2 - Optical Communication Systems
Characteristics and Performance
Aim:
The aim of this laboratory exercise is to study and understand the characteristics and limitations
of optical communication systems. Concepts such as Signal-to-Noise-Ratio (SNR), Bit-Error-
Rate (BER), receiver sensitivity and power penalty should be understood.
Before the lab:
1) Read this PM a few days before the laboratory exercise in order to be able to ask questions to
your supervisor if necessary.
2) Solve the theoretical exercises marked with #1-5. Students that do not attempt seriously
in solving these exercises will be sent away from the laboratory exercise.
3) Read the following subchapters in Fiber-Optic Communication Systems (Edition 3): 4.3-4.7, 6.1.3
and 6.5.1 (skip 4.3.4, 4.4.3 and 4.6.3). These sections totals only 26 pages with essential
information about the lab that is also important for the exam.
Name: ___________________________________________
Date: ____________________________________________
Teacher’s signature: ________________________________
Introduction In this laboratory exercise we will study the performance of optical communication systems with the emphasis put on the receiver performance. The lab is divided into three parts. The first part deals with the structure and the performance of optical receivers. In the second part, we study how an optical receiver can be characterized by bit-error-rate measurements in terms of receiver sensitivity and power penalty due to different signal distortions. Last but not least we investigate the performance of a simple fiber-optic communication system experimentally.
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1 Optical receivers
1.1 Receiver components The role of the optical receiver is to convert the received optical signal into an electrical form and to recover the data. Fig. 1 illustrates the typical structure of a digital optical receiver. The optical receiver can be divided into three principal blocks: the front end, the linear channel and the data recovery. Furthermore, the front end is sometimes preceded by an optical preamplifier (the framed part of Fig. 1).
Figure 1. The typical structure of a digital optical receiver.
1.1.1 Front end The front end of the optical receiver consists of a photodetector and an electrical preamplifier. The photodetector can be considered as the main receiver component since it converts the received optical signal into an electrical one and makes electronic data processing possible. The photodetectors used in most communication systems are reverse biased p-n junctions (photodiodes) of mainly two different types: p-i-n photodiodes and avalanche photodiodes (APD). The APD has a higher responsivity (R) than the p-i-n photodiode but it is also associated with higher noise and smaller bandwidth. When required the receiver sensitivity can be improved by optically amplifying the data signal before photdetection (shown in Fig. 1). The most common optical amplifier used for this purpose is the Erbium-doped fiber amplifier, EDFA. However, optical amplification is always associated with increased noise (spontaneous emission). To reduce the amplifier noise it is very advisable to filter the amplified signal using an optical bandpass filter (Fig. 1). In this way the useful part of the amplified signal can be extracted and detected. The last step of the front end consists of an electrical preamplifier that amplifies the detected signal for further data processing. In this laboratory exercise we will consider an optical receiver containing a p-i-n photodiode preceded by an EDFA and an optical bandpass filter.
1.1.2 Linear channel The linear channel of the optical receiver consists of a high-gain amplifier with an automatic gain control and a lowpass filter. The gain of the amplifier is controlled to provide a constant average signal voltage at the output of the linear channel irrespective of the average input voltage. This
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simplifies the construction and improves the performance of the data recovery part of the receiver. The lowpass filter is used to reduce the signal noise. The noise superposed on the signal can be assumed to have a constant spectral density (i.e. the noise can be assumed to be ¨white¨) and thus the amount of noise will be proportional to the bandwidth of the lowpass filter. On the other hand, lowpass filtering will broaden the signal pulses and too strong filtering will cause intersymbol interference (ISI, see section 2.1). The amount of ISI is inversely proportional to the filter bandwidth. Consequently, the bandwidth of the lowpass filter should be chosen to reduce the noise without initiating too much ISI.
1.1.3 Data recovery The data recovery section of optical receivers consists of a clock-recovery circuit and a decision circuit (Fig. 1). The clock-recovery circuit is used to obtain a spectral component at f = B from the received signal, where B is the data rate. This component provides information about the bit slot and enables proper synchronization of the decision circuit. The process of clock-recovery is accomplished in different ways depending on the format of the data being transmitted. In the case of Return-to-Zero (RZ) format, the data already contains a spectral component at f = B, and thus, clock-recovery is easily done by filtering the received signal using a narrow-band bandpass filter. For Non-Return-to-Zero (NRZ) format however, clock-recovery is more complicated as the received signal lacks a spectral component at f = B in this case. A common technique to solve this problem is to use a full-wave rectifier to convert the NRZ data to a RZ waveform containing a delta function at f = B. The decision circuit compares the output voltage from the linear channel to a pre-defined threshold level, at sampling times determined by the clock-recovery circuit, and decides whether the received pulse corresponds to a ¨1¨ or a ¨0¨. Both the threshold value and the sampling times can be optimized to minimize the error probability of the decision circuit. The eye-diagram, achieved by superposing the received electrical pulses on top of each other, is a practical way of determining the best sampling time for the decision circuit. In order to minimize the error probability the sampling time should be chosen where the opening of the eye-diagram is at its maximum.
1.2 Receiver noise The conversion of incident optical power into electrical current in an optical receiver is not a noise free process. Even a perfect optical receiver suffers two fundamental noise mechanisms: shot noise (section 1.2.1) and thermal noise (section 1.2.2). If the receiver contains an optical preamplifier, the spontaneous-emission noise of the amplifier (section 1.2.3) should also be taken into account. Due to the receiver noise the detector current experiences time fluctuations even if the incident optical signal has a constant power. The detector current, I(t), can be related to the incident optical power, Pin, as: I(t) = Ip+ is(t) + iT(t) + isp(t), (1) where Ip = RPin is the average current. is(t), iT(t) and isp(t) represent the current fluctuations caused by shot noise, thermal noise and spontaneous-emission noise, respectively.
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1.2.1 Shot noise Shot noise is a result of the randomness of the photocurrent generation, which stems from the fact that the number of photons arriving at the detector in each time interval is a stationary random process with Poisson statistics. Hence, the photocurrent will fluctuate as a function of time even for a constant average optical power. Furthermore, shot noise is approximately ¨white¨ having a (one-sided) spectral density given by Ss(f) = 2qIp and a variance given by:
!2
s = i
2
s (t) = 2qIp"f = 2qRPin"f (2)
where q is the elementary electron charge and Δf is the effective noise bandwidth of the receiver, often determined by the lowpass filter in the linear channel of the receiver. The quantity σs is the rms value of the shot noise induced current, is(t).
1.2.2 Thermal noise Random thermal motion of electrons in the load resistor in the front end of an optical receiver add some fluctuations, iT(t), to the photocurrent. This noise component is referred to as thermal noise. Thermal noise is a stationary Gaussian random process with nearly ¨white¨ spectral density: for frequencies up to f≈1 THz, the (one-sided) spectral density of thermal noise is constant given by:
S T(f) =
4kB T
RL . (3) where kB is the Boltzmann constant, T is the temperature in Kelvin and RL is the load resistance in the front end of the receiver. In excess of the load resistance in the front end, electrical amplifiers used in the receiver add thermal noise, which is quantified by the amplifier noise figure, Fn (ideally Fn = 1 (0 dB)). Hence, the total thermal noise variance is then calculated as:
!
2
T =
4kB
TFn"f
RL (4)
1.2.3 Spontaneous-emission noise Optical amplification is always associated with increased noise because of the spontaneous emission that adds to the signal during its amplification. This spontaneous-emission noise leads to a degradation of the Signal-to-Noise-Ratio, SNR. As will be discussed in more details in section 1.3, the SNR is the ratio between the average signal power and the noise power. The SNR degradation due to the spontaneous-emission noise is quantified through the amplifier noise figure, Fn, as:
F n =
(SNR)in
(SNR)out (5)
5
where SNRin/SNRout refers to the SNR at the input and the output of the amplifier, respectively. If the gain, G, of the optical amplifier is large enough, Fn can be calculated according to: Fn = 2 nsp (G-1) / G ≈ 2 nsp (6) where nsp is the population inversion factor. For an optical amplifier with a two-level system, which is the case for EDFA:s pumped at λp = 1480 nm, nsp can be calculated as: nsp = N2 / (N2 - N1) (7) where N1 and N2 are the atomic populations for the ground and the excited states, respectively. From Eqs. 6 and 7 it can be understood that the SNR of the amplified signal is degraded by a factor of 2 (3 dB) even for an ideal amplifier having nsp = 1 (N1 = 0). However, for most practical amplifiers Fn is as large as 6-8 dB. Spontaneous emission spans a wide optical frequency range, Δνsp. Consequently, spontaneous emission can beat against itself and the amplified signal. The variance, σ sp
2, of current fluctuations, isp(t), due to the spontaneous emission is mainly composed of two dominant noise terms originating from the beatings of the spontaneous emission against itself, σ sp-sp
2, and against the signal, σ sig-sp
2:
!2
sp = i
2
sp(t) = !2
sig" sp+ !
2
sp" sp (8) where
!2
sp" sp= (q#GFn )
2 $%
opt$f (9)
!2
sig" sp= 2 (q#G)
2 Fn P in$f /h%
, (10) where η is the detector quantum efficiency, ν is the center frequency of the incident optical signal and Δνopt is the optical bandwidth of the spontaneous-emission noise. In the absence of an optical bandpass filter after the amplifier, Δνopt is equal to Δνsp. However, if the amplified signal is filtered, Δνopt is reduced and is determined by the bandwidth of the bandpass filter. The minimum optical bandwidth is equal to the bit rate (Δνopt = B). If it can be assumed that a logical ¨0¨ does not contain any optical power, the spontaneous-emission noise currents for a logical ¨1¨, σ 1, and a logical ¨0¨, σ 0, can be expressed as:
!
1= (!
2
sig" sp+ !
2
sp" sp)
1 / 2 , !
0= !
sp -sp (11) For an alternative explanation of the beat noise phenomenon see Fig. 2.
6
Figure 2: Consider two noise components (or one noise and one signal component) with slightly different optical frequencies ν1 and ν2. The beating between the two component (sin(ν1t)+sin(ν2t)=2sin((ν1+ν2)t/2)cos((ν1-ν2)t/2))) will give rise to a very rapid oscillation, but also a slowly varying envelop with a frequency (ν1-ν2)/2. When this optical field is detected by a receiver with limited bandwidth, Δf, the rapid oscillation will be filtered away, but the beating term will give rise to a noise current if (ν1-ν2)/2< Δf. Hence, in the case of signal-spontaneous beat noise only ASE-noise components within Δf around the signal wavelength will contribute to the noise variance. However, spontaneous-spontaneous beat noise will have contributions from the whole noise bandwidth. By putting an optical filter after the EDFA the amount of spontaneous-spontaneous beat noise can be reduced, but the signal-spontaneous beat noise will remain the same.
1.2.4 Noise level comparison In order to compare the amount of noise originating from the different noise sources presented in the previous subsections we calculate an example based on realistic experimental data. The noise levels are then shown in Fig. 3 as a function of average optical input power. First, consider a receiver without optical pre-amplification. The detected 3 Gb/s data signal was for simplicity considered to have no power in the logical ¨0¨. Therefore, Prec = P1 / 2, where P1 is the optical power in a ¨1¨ (assuming 50 % ones). The other relevant receiver parameters are given in the caption of Fig. 3. The broken line in Fig. 3 shows the variance of the shot noise, σ s
2, for a logical ¨1¨ versus the average received optical power, Prec (Eq. 2). In Exercise #2 the thermal noise variance for the same system was calculated and entered into the same figure. Can any conclusions be drawn on which noise source that will limit the system when not using optical pre-amplification? The spontaneous-emission noise in a ¨1¨, σ 1
2, for the system discussed in the previous sections is plotted as a function of the average received optical power, Prec (= P1 / 2), in Fig. 3 (dotted line). The amplifier considered has G = 30 dB and Fn = 5 dB and is followed by an optical bandpass filter resulting in Δνopt = B. As can be seen in Fig. 3, spontaneous-emission noise dominates over thermal noise and shot noise for all power levels shown. However, the shot noise level shown in Fig. 3 is calculated for an optical receiver without an optical preamplifier. In the presence of an optical preamplifier, the variance of the shot noise is increased by a factor of G (= 30 dB = 1000 in the example shown in Fig. 3). In spite of this increase, the shot noise will still be much less than the spontaneous-emission noise in the example shown in Fig. 3. In fact, this is often the case in realistic systems.
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Figure 3. Variances of shot noise (broken line) and spontaneous-emission noise (dotted line) for a 3 GHz optical receiver. A logical ¨0¨ has been assumed not to carry any optical power. The other system parameters are: λ = 1.55 µm, R = 1 A/W (η = 80%), Δf = Δνopt = B = 3 GHz. The gain of the optical amplifier, G = 30 dB and the optical amplifier noise figure, Fn = 5dB.
1.3 Receiver performance To discuss the performance of an optical receiver we need to introduce some tools to measure the performance with: - The Signal-to-Noise-Ratio, SNR, is defined as:
SNR =
Average signal power
Noise power =
I2
p
!2
(12) In the case of an optical receiver containing a p-i-n diode preceded by an EDFA, the SNR of the receiver can be calculated as:
SNR =
I2
p
!2
T+ !
2
s+ !
2
sig" sp+ !
2
sp" sp (13) - The Bit-Error-Rate, BER, is the probability of incorrect bit identification by the decision circuit of the receiver. BER is the most useful criterion to describe the transmission quality of a digital communication system. A typical requirement for optical receivers is BER < 10-9. With equal occurrence probabilities of logical ¨1¨:s and ¨0¨:s and Gaussian noise, the BER is given by:
8
!!"
#
$$%
&
''
(
)
**
+
, -+''
(
)
**
+
, -=
2ó
IIerfc
2ó
IIerfc
4
1BER
0
0D
1
D1 (14)
where I1 and I0 are the average signal currents at the input of the decision circuit for a ¨1¨ and ¨0¨, respectively. σ 1 and σ 0 are the rms noise currents for a ¨1¨ and ¨0¨, respectively. ID is the threshold current value of the decision circuit. An adequate choice of ID is:
I
D=
!0I
1+ !
1I
0
!1+ !
0 . (15) Using Eq. 15, Eq. 14 can be re-written as:
!"
#$%
&=
2
Qerfc2
1BER where
01
01
óó
IIQ
+
!= (16)
Hence, a BER < 10-9 requires that Q > 6. - Receiver sensitivity is defined as the minimum average optical power at the receiver required to achieve a certain BER. Since it is more practical to measure the received optical power than Q, it is more common to specify the performance of a communication system by determining the required receiver sensitivity rather than the value of Q. In the following three sections we will discuss the performance of an optical receiver in a system limited by either shot noise, thermal noise or spontaneous-emission noise. These three sources of noise are uncorrelated to each other and thus the effects of them can be added to each other whenever more than one of them are present. Consequently, we do not lose any of the generality by studying the effects of each of these noise sources alone. Throughout these sections we consistently consider a case where a ¨0¨ has no optical power, i.e. I0 = 0.
1.3.1 Performance of a thermally limited system When thermal noise is the dominant noise source in an optical receiver, Q can be expressed as:
Q =
I1
!0+ !
1
= "1
2!T
= RP
1
2!T
= RP
rec
!T (17)
Using Eq. 16, BER can be related to the average received power, Prec:
!!
"
#
$$
%
&=
T
rec
ó2
RPerfc2
1BER (18)
Fig. 4 illustrates BER versus Prec for the system discussed in section 1.2.4 with thermal noise as the only noise. Prec is measured in µW in this figure. However, when discussing optical communication systems it is more common to measure the optical power in dBm. Furthermore,
9
when studying graphic representations, it is desirable to have a graphically linear relation between BER and Prec. This makes it easier to study the characteristics of the resulting graphs such as BER floors and curve slopes. Consequently, it is common to illustrate the relation between BER and Prec using a graph with a special logarithmic BER axis as shown in Fig. 5 (solid line).
BER
10-2
10-14
10-12
10-10
10-8
10-6
10-4
5 6 7 8 9 10 11 12 13 14 15
Average optical input power, P rec [µW]
Figure 4. BER versus Prec for a thermally limited system having the same parameters as in section 1.2.4.
-55 -50 -45 -40 -35 -30 -25 -20 -15
Average optical input power, Prec [dBm]
10-4
10-14
10-10
10-9
10-8
10-7
10-6
10-5
BER
Figure 5. BER versus Prec for a system limited by thermal noise (solid line), shot noise (dotted line) and spontaneous-emission noise (broken line). The system parameters are the same as in section 1.2 (see the caption of Fig. 3).
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1.3.2 Performance of a shot noise limited system In the absence of optical power in a logical ¨0¨, σ 0 = 0 in a shot noise limited system (Eq. 2). Consequently, Q can be expressed as:
Q=
I1
!1
="1
!s
=RP1
2q RP1#f=
RPrec
q#f (19)
Using Eq. 16 BER can now be calculated:
BER =
1
2erfc
RPrec
2q!f (20) In Fig. 5 the BER is shown versus Prec for the system discussed in the previous section but this time with the shot noise as the limiting source of noise (dotted line). When the number of incident photons is very small (<100), the Gaussian approximation of shot noise is not valid but the exact Poisson statistics must be applied. If Np is the average number of photons in a logical ¨1¨, the actual number of generated electron-hole pairs in the photodetector will fluctuate from the average value Np according to the Poisson distribution:
P m =
e!N
p Nm
p
m! (21) where Pm is the probability of generating m e-h pairs for a logical ¨1¨. With an ideal detector (σ T = 0 and η = 1) and assuming I 0 = 0 (i.e. no optical power in a logical ¨0¨), the threshold of the decision circuit can be set at one photon. Errors will then appear whenever a ¨1¨ generates no electron-hole pairs and is interpreted as a ¨0¨: BER = 1/2 P(0/1) = 1/2 Pm=0 = {Eq. 21}= 1/2 e-Np (22) where P(0/1) is the probability that a sent ¨1¨ is interpreted as a ¨0¨ by the decision circuit. Thus, for BER < 10-9, Np must exceed 20. Since this requirement is a direct consequence of the quantum fluctuations connected to the incoming light it is referred to as the quantum limit of photodetection.
1.3.3 Performance of a system limited by spontaneous-emission noise As discussed in section 1.2.3 spontaneous-emission noise dominates over thermal and shot noise in most optically preamplified receivers. For such systems, Q can be related to the average input optical power, Prec, as:
Q = !
1
2
"#opt
"f+
"#opt
4 "f +
P rec
h#F n"f (23)
11
Using Eq. 16 BER can now be related to Prec:
!!
"
#
$$
%
&
!!
"
#
$$
%
&++'=
Äfhõõ
P
Äf4
Äõ
Äf
Äõ
2
1
2
1erfc2
1BER
n
recoptopt (24)
In Fig. 5 BER is illustrated as a function of Prec based on Eq. 24 (broken line). The system parameters are the same as in section 1.2.4 (see the caption of Fig. 3). As discussed in section 1.2.4, spontaneous-emission noise dominates over thermal and shot noise. Consequently, the receiver sensitivity calculated according to Eq. 24 can be considered to be the total receiver sensitivity for the system, taking all three sources of noise into consideration.
12
2 Receiver sensitivity degradation in fiber-optic communication systems
The receiver sensitivity analysis in the previous sections was based on the assumption that the pulses were ideal, corrupted only by receiver noise. In reality, the receiver sensitivity is degraded due to the optical transmitter and the optical fiber. The corresponding necessary increase in average received optical power to maintain a certain BER is called the power penalty. Optical transmitters cause power penalty through a non-zero extinction ratio (i.e. a non-zero power content in a sent ¨0¨) and intensity noise (i.e. time fluctuations of the emitted optical power). The power penalty associated with the pulse transmission through the optical fiber originates from the fiber dispersion and is discussed in the following section.
2.1 Power penalty due to pulse transmission through optical fibers When optical pulses propagate through an optical fiber, they are broadened as a result of the fiber dispersion. Dispersion-induced pulse broadening affects the receiver performance in mainly two ways: 1- A part of the pulse energy spreads outside the designated bit slot and causes intersymbol interference (ISI). ISI is pulse shape dependent and is minimized when the pulses have a raised-cosine spectrum. To keep the ISI within reasonable bounds for pulses with arbitrary shape, a commonly used criterion is that the rms pulse width should not exceed 25% of the bit slot. For Gaussian pulses this criterion means that at least 95% of the pulse energy remains within the bit slot. 2- The spread of the optical energy beyond the bit slot reduces the pulse energy within the bit slot. Consequently, the signal SNR at the decision circuit is also reduced. In order to maintain the system performance, this SNR reduction should be compensated for by increasing the average optical power. This is the origin of dispersion-induced power penalty.
Figure 6: Illustration of the dispersive degradation of the transmitted data. The result is a power penalty to the receiver sensitivity due to energy in the neighboring bit-slots (ISI) and reduced SNR due to reduced peak signal power.
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3 Experiments In this section we will study the performance of a 3 GHz fiber-optic communication system experimentally. The system performance is measured as the minimum average optical power at the receiver necessary to obtain a certain BER. Optical pseudo-random bit sequences (PRBS) of NRZ rectangular pulses are transmitted through the system and the BER is measured at the receiver for different average received powers. In reality, the pulse sequences transmitted through a communication system are of course truly random. However, in laboratory environments it is very common to use PRBS:s to simulate reality. PRBS:s have correlation and spectral characteristics that resemble those of a random sequence and thus they are appropriate to be used when simulating truly random sequences. In this laboratory exercise we will carry out the following experiments: In experiment 1a we will study the performance of the so-called back-to-back structure of the communication system, i.e. when the optical transmitter is directly connected to the optical receiver. The results of this first experiment will give us some information about the quality of the transmitter and the receiver used in the system. The set-up of experiment 1b is very much like that of experiment 1a except that now the optical pulses are propagated through an optical fiber before they reach the receiver. We investigate how much the system performance is deteriorated due to the fiber transmission. In experiment 2 we will examine how the system performance is affected by optical preamplification at the receiver. An EDFA is used as an amplifier. We will examine the performance of the back-to-back system including an EDFA at the receiver both with (experiment 2a) and without (experiment 2b) an optical bandpass filter. Finally, in experiment 2c, we will consider the case when the pulses are propagated through an optical fiber and investigate how much the system performance is improved by using an EDFA followed by an optical bandpass filter at the receiver. Experimental Setup The experimental set-up is shown in Fig. 7. The optical transmitter being used is a distributed feedback (DFB) semiconductor laser having an operating wavelength λ = 1.55 µm. The laser is biased by a DC current source and modulated by a PRBS generator. The PRBS generator can produce PRBS:s of 27-1 up to 231-1 NRZ rectangular pulses with repetition frequencies up to 3 GHz. Furthermore, the amplitude of the pulses can be varied between 0.5 V and 2.0 V. Optical feedbacks into the laser may damage it and should therefore be prevented. For this purpose an optical isolator is mounted at the laser output. Moreover, the transmitted optical power is regulated using a variable optical attenuator. To measure the received optical power, an inline optical power meter is used to monitor the signal power entering the receiver, Prec. The simplest front end of the receiver we investigate consists of a p-i-n photodiode followed by an electrical preamplifier (Lab. Ass. 1). In order to supply the error counter with a high enough and constant voltage a linear channel based on electrical amplifiers precedes the error counter. With a constant average signal voltage at the error counter input, the resulting BER only depends on the SNR of the received optical signal and not on the voltage dependence of the performance of different electronic components in the error counter. After the electrical amplification in the linear channel the signal is filtered by a Bessel lowpass filter having a bandwidth of Δf = 2.5 GHz. Finally, the filtered signal enters the error counter and a sampling oscilloscope using an electrical 10 dB tap. The sampling oscilloscope, which is used to
14
observe the detected pulses and the corresponding eye diagram, is also provided with a trigger signal from the PRBS generator, as shown in Fig. 7. The error counter compares the received pulses with the PRBS that was actually sent. The result of this comparison is presented as the BER. The threshold voltage and the sampling time of the error counter can be determined both manually and automatically. To be sure that the optimum choices have really been made, it is advisable to regulate these parameters manually until the presented BER is minimized. To operate properly, the error counter requires a signal voltage amplitude, Vp-p, of 0.5 V - 2V and a clock signal. In reality, the clock signal is extracted from the received data. However, for simplicity, the clock signal is collected directly from the PRBS generator in this laboratory exercise. For moderate propagation distances, this technique works properly. In lab assignment 2 we will improve the front end by adding optical pre-amplification by an EDFA and optical filtering by an optical bandpass filter. These experimental setups are also shown in Fig. 7 as alternative paths in the front end. Note that in this assignment the second power meter should be kept at a constant power level above the thermal limit. Do you understand why?
Figure 7: Experimental setup for all experimental assignments. The dashed lines together with the assignment numbers indicate the different setup that we are to investigate. In 1a the optical transmitter is directly connected to the optical receiver (the so-called back-to-back configuration). In 1b the optical pulses are propagated through a standard single mode fiber before they reach the receiver. In assignment 2 a preamplified front end is investigated with (2a,2c) and without (2b) an optical bandpass filter (O-BPF). In 2c the fiber is hooked up again.
15
3.1 Experiment 1 : Thermally limited receiver Lab Assignment 1a: Back-to-Back First, please let your lab assistant introduce you to the different components used in the experiment. Most of these components are very sensitive and expensive and should therefore be handled very carefully. Setup the transmitter: In order to achieve a high quality optical data signal the DFB laser needs to be fed by the correct drive current. Hence, we need to find the optimum bias point of the laser. Start by measuring the threshold current of the DFB laser. Disconnect the receiver from the transmitter and use the bias current supply and the optical power meter to determine the laser threshold current. The DFB laser threshold current, Ith = _______ mA. Use the following parameters for the PRBS generator: Repetition frequency = 3 GHz PRBS word length = 215-1 Pulse amplitude, Vp-p = 0.7 V (50 Ω load) Pulse offset voltage = 0V
Figure 8: DFB laser characteristics and schematic modulation setup
If the maximum allowable laser current is Imax=50 mA, calculate the maximum allowable bias current, Ib,max , when the laser is being modulated by a PRBS generator having the above parameters? The laser is terminated with a load resistance of 50 Ω . Also discuss the optimum choice of the bias current for semiconductor lasers as transmitters in communication systems. ___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
Maximum laser bias current, Ib,max =_________ mA. Bias the laser below Ib,max. Now the optical receiver is to be connected to the transmitter. Before you do so, use the variable optical attenuator to reduce the signal power into the photodiode below the maximum allowable level of -10 dBm.
Allways disconnect the receiver
before changing anything in the setup!!!
16
The trigger signal to the oscilloscope is collected from the PRBS generator contact marked ¨PATTERN¨ or ¨1/16 CLOCK¨ . Investigate when each of these two trigger signals should be used. What is the difference? ______________________________________________________________________________________________________________________________________________________ Now measure the BER versus average input power to the optical receiver, Prec, for the system. Use the optical attenuator at the transmitter to vary the optical power and be careful so that the optical power into the receiver never exceeds -10 dBm (the maximum tolerable photodiode power). Adjust the threshold value and the sampling time of the error counter manually to minimize the resulting BER for each case. Register your measurements in table 1 and plot the BER versus Prec in the graph given in Fig. 9. Now use this graph to calculate Prec necessary to obtain BER =10-9 and register the result in table 2. How many photons/bit does this correspond to? Assume that the system is thermally limited and compare your experimental results with your calculations in exercise # 3. To accomplish this comparison use the following parameters for the p-i-n photodiode used in the experiment: R = 0.27 A/W and RL = 50 Ω. ___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
Average received optical power Prec [dBm]
Bit-Error-Rate BER
Table 1. BER versus Prec for the back-to-back structure of the system.
17
Figure 9. BER measured versus Prec.
Lab Assignment 1b: Fiber impact on the receiver sensitivity Insert the optical fiber into the system as shown in Fig. 7. Investigate how large the average received optical power, Prec, needs to be to obtain a BER of 10-9. Register your result in table 2 and use the result achieved in the previous experiment to discuss the receiver sensitivity degradation caused by the optical fiber. How large is the dispersion induced power penalty at BER=10-9? Explain how the eye-diagram is affected by the dispersion and which effect that mainly contributes to the penalty. ___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
18
Discuss how an increase of the data rate would affect the receiver sensitivity degradation caused by the optical fiber. ___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
System configuration Prec [dBm] Back-to-back Transmission through SMF fiber Back-to-back with preamplification and optical bandpass filtering at the receiver
Back-to-back with preamplification at the receiver (without the optical bandpass filter)
Transmission through SMF fiber with preamplification and optical bandpass filtering at the receiver
Table 2. Prec necessary for BER = 10-9 for different system configurations.
3.2 Experiment 2
Fig. 7 illustrates the set-up of the experiment using the alternative paths in the front end. The received signal is now amplified by an EDFA and filtered by an optical bandpass filter before it enters the detector. The EDFA consists of an about 20 m long Erbium-doped fiber pumped at either 980 nm or 1480 nm. The EDFA pump power level is adjusted by changing the bias current of the pump laser. This current must not exceed 200 mA. The optical bandpass filter being used has an adjustable center wavelength around 1560 nm and a bandwidth of 1 nm. The optical power into the p-i-n photodiode is kept constant using the variable optical attenuator after the EDFA, as shown in Fig. 7. Lab Assignment 2a:
The BER is to be measured versus average optical power, Prec, received at the EDFA input (input of the front end). Now vary the transmitted optical power using the variable optical attenuator at the transmitter and study the resulting BER at the receiver. The center wavelength of the optical bandpass filter should be adjusted to minimize the BER. The aim of this experiment is to study the system characteristics limited by the EDFA performance only. Consequently, the optical power into the p-i-n photodiode should be held at a constant and sufficiently high level so that the achieved BER is not limited by the photodiode performance. Use the second power meter and attenuator to keep the input optical power to the p-i-n photodiode at a constant average power of slightly below -10 dBm. At this input power level the p-i-n photodiode does not contribute to the BER (see the result achieved in experiment 1a).
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Register your measurements in table 3 and plot the BER versus Prec in the graph of Fig. 9. Now use this graph to calculate Prec necessary to obtain BER =10-9 and register the result in table 2. How many photons/bit does this correspond to? Compare the achieved result with that of the previous experiments. Does this experimental result agree with the theory discussed in section 1.3.3 and your calculations in exercise #4? If it does not, discuss why. The noise figure of the EDFA used in the experiment is approximately 6 dB. ___________________________________________________________________________
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Average received optical power Prec [dBm]
Bit-Error-Rate BER
Table 3. BER versus Prec for the back-to-back structure of the system, including an EDFA and an optical bandpass
filter.
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Lab Assignment 2b: The effect of the bandpass filter
Remove the optical bandpass filter after the EDFA and investigate how large the average received optical power, Prec, needs to be to obtain a BER of 10-9. Register your result in table 2 and compare it with the case where the optical bandpass filter is included in the receiver. Please note that if this comparison is to be fair, the p-i-n photodiode should be provided with approximately the same average power and the EDFA pump current should be the same as in experiment 2a. Also compare your result with the case when the system has a back-to-back structure without any preamplifier at the receiver (i.e. Experiment 1a). ___________________________________________________________________________
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___________________________________________________________________________ Lab Assignment 2c: Complete fiber optic system Place the optical bandpass filter back in the receiver and insert the optical fiber into the system, as shown in Fig. 10. Investigate now how large the average received optical power, Prec, needs to for a BER of 10-9 and register your result in table 2. The EDFA pump current and the average optical power at the input of the p-i-n photodetector should be the same as in the previous experiments. The center wavelength of the optical bandpass filter should also be adjusted to minimize the BER. How large is the dispersion-induced power penalty in this experiment? Also compare your result with the results achieved in the previous experiments. Discuss the results given in table 2. ___________________________________________________________________________
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4 Mandatory Exercises Exercise #1: For proper performance the data recovery part of an optical receiver requires an average input voltage of about 1 V. The received optical signal has a center wavelength, λ, of 1.55 µm and an average power, Prec, of -40 dBm. The received signal is amplified by an EDFA before it is detected by a p-i-n photodiode. The EDFA gain is 30 dB at this level of incident power. The p-i-n diode being used has a quantum efficiency, η , of 80% and a load resistor, RL, of 50 Ω . Calculate (in dB) how much the detected signal needs to be amplified before it enters the data recovery circuits of the receiver. For simplicity assume that no signal power is lost in the receiver filters. Answer #1: Exercise #2: Calculate the thermal noise variance for the receiver considered in section 1.2.1 and Fig. 3. The load resistor, RL= 50 Ω . Assume room temperature and ideal electrical amplifiers (Fn = 0 dB). Mark now the thermal noise level in Fig. 3. Answer #2: Exercise #3: Start with Eq. 22 and derive an expression for the BER in terms of the average input optical power, Prec, the bit rate, B, and the center frequency of the optical signal, ν, at the quantum limit of photodetection. Mark the quantum limit for the system discussed in this section in Fig. 5. Use now Fig. 5 to calculate how much Np needs to increase (compared to the quantum limit) when the system performance is limited by shot noise/thermal noise if BER is required to be at least 10-9. Answer #3:
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Exercise #4: Use Fig. 5 to calculate the average number of photons/bit for the spontaneous-emission noise limited system if a BER < 10-9 is required. Answer 4: Exercise #5: Explain briefly the following: Power penalty: Back-to-back structure: Pseudo-Random Bit Sequence (PRBS): How to find the receiver sensitivity from a back-to-back BER curve: Extinction ratio: Eye-diagrams: ___________________________________________________________________________
